Process for recovery of silica followed by alumina from coal fly ash

ABSTRACT

A process for recovery of silica and alumina from fly ash, in which, Si is leached out from the fly ash in the form of Na 2 SiO 3  using a NaOH solution of NaOH &gt;40% (w/w), followed by separation to obtain a Na 2 SiO 3  solution and a residue having Al—Si ratio ≧2; vaporizing the solution to obtain Na 2 SiO 3  solutions of various concentrations, or producing silica by carbonation, and producing Al 2 O 3  by prior art methods from residues obtained after alkali-leaching, ultimately producing filler or cement from the residue after the recovery of Al. The method permits recovery of Al 2 O 3  directly from fly ash by removing Si before recovering the Al, which raises the Al—Si ratio of the residue after alkali-leaching and thus simplifies the recovery of Al 2 O 3  and raises the rate of Al recovery from fly ash. The invention further includes calcining high Al coal gangue, kaolinite and middle-low-class bauxite at 900-1100° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a 371 national stage application of international application no. PCT/CN2007/002475, filed Aug. 16, 2007, which claims priority to Chinese application No. 200710061662.X, filed Apr. 3, 2007, and Chinese application No. 200710062534.7, filed Aug. 7, 2007, each of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention refers to fine utilization of fly ash, and particularly to a process for recovery of silica and alumina from fly ash, and more specifically refers to a process in which silica is firstly recovered from fly ash, remaining an Al—Si ratio ≧2, then metallurgical alumina is produced with prior art, the residue of which is used as a filler or to produce cement.

BACKGROUND OF THE INVENTION

Large amounts of fly ash are discharged from power plants in which coal is burned that causes severe pollution and damage to agricultural and natural ecology around the plants. It is therefore now a project that needs to be solved urgently to utilize the fly ash.

Fly ash is also a mineral resource. It generally contains about 15-40% Al₂O₃ and mostly above 40% SiO₂, even above 40% Al₂O₃ and about 50% SiO₂ in high Al fly ash. More than 3 hundred million tons of fly ash are discharged from lire power electrical plant each year in China, wherein 1 hundred million ton is no less than high aluminum fly ash. If resources from high aluminum fly ash that is being abandoned can be fully recovered, more than 30 million tons of Al₂O₃ can be produced each year, which is far higher than the total production of Al₂O₃ today in China Bauxite stores in China are only 1/10 of the world average stores. It is therefore significant and important for the sustainable development of an aluminum industry in China to develop and use the resources of high Al fly ash.

The method of recovering Al₂O₃ from fly ash can be divided into acid methods and alkali methods. With acid processing, damage to SiO₂ can be avoided while Al oxide is efficiently recovered from fly ash. But when leaching Al₂O₃, the disadvantage of acid processing is that a number of soluble impurities such as Fe, Ti, Mg contained in the fly ash are introduced into the solution, so post-treatment must be added; another disadvantage is that apparatus of the acid method needs better acid-corrosion-resistance, so it is rather difficult to make the reaction facility; Another disadvantage regarding the acid process for recovery of Al₂O₃ from fly ash includes high energy consumption and the necessary cost for environmental protection.

In the 1960's, a soda lime sintering method was used to recover Al₂O₃ from fly ash in Poland and an experimental plant which produced ten thousand tons of Al₂O₃ and 100 thousand tons of cement each year was built up there. In the 1980′s, Metallurgy Institute of Anhui Province and Hefei Cement Institute in China declared the achievement of recovering Al₂O₃ from fly ash by sintering limestone and dissolving Na₂CO₃ and producing cement with the residue. This achievement passed the expert examination in March 1982. The process of recovering Al₂O₃ from fly ash by soda lime sintering, and producing cement with the residue studied by Building Material Institute of Ningxia Autonomous District was examined by Technology Committee of Ningxia Autonomous District in September 1987. The examination of the project entitled “Industrialization of recovery Al₂O₃ and producing cement from fly ash”, which was researched and developed by Mengxi high-novel technology group LTD., was held in Inner Mongolia Autonomous Region S. & T. Department in December 2004, and a pilot study of nearly 5000 ton-class was completed by the group itself. But the process of treating high Si and Al-containing fly ash by alkali methods normally has such disadvantages as high complexity, high time consumption, large quantities of processing materials, large investment of facility, high energy consumption and high cost. Furthermore the amount of residue is several times that of fly ash; the market coverage of cement made from the residue is limited; the comprehensive economic efficiency and the level of synthetic utilization are low All of these factors inhibit the use of alkali methods in synthetic utilization of fly ash.

Today 90% of the Al₂O₃ in the world is produced by the Bayer Process. But the Bayer Process requires a relatively high Al—Si ratio i.e. ≧7 in raw ore. In order to produce Al₂O₃ with bauxite of 3<Al—Si-ratio<7, which is economically unfavorable if Bayer Process is directly used, a sintering treatment of ore powder is needed with added alkali and Calcium or complex methods to recover Al₂O₃; resulting in $20-50/ton higher cost than that of the Bayer Process. The Al—Si ratio of fly ash is generally less than 1, which is far from the conditions on which industry-facilities using the Bayer Process can be used to recover Al₂O₃ directly.

SUMMARY OF THE INVENTION

An object of the present invention to provide a process for recovering silica and alumina from fly ash, in which silica is firstly recovered from the fly ash to give a residue having an Al—Si ratio ≧2, then metallurgical alumina is produced using conventional methods. A residue can be used to produce filler or cement.

Another object of the invention is to provide a process for recovering silica and alumina from coal fly ash, comprising: a) alkali-leaching Si from fly ash at 70-150° C. with a NaOH solution of concentration >40% (w/w), then separating a Na₂SiO₃ solution from a residue having Al—Si ratio ≧2; b) concentrating the Na₂SiO₃ solution by vaporization, or obtaining silica by carbonation; and c) producing Al₂O₃ from the residue having Al—Si ratio ≧2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Activation of fly ash can be done by roasting, acid-steeping or alkali-steeping. Suitable roasting conditions are roast-activation for 1-1.5 h at a temperature of ≦950° C. H₂SO₄ solution of any concentration can be used to steep the fly ash, and either under ambient or controlled temperature. On the other hand, a suitable alkali solution used to steep fly ash is 5-20% (w/w) NaOH solution.

Conventional methods used to produce Al₂O₃ from the residue obtained after alkali-leaching include soda lime sintering methods or limestone sintering methods, in which Al(OH)₃ is produced by clinker-sintering, clinker-leaching-out, desilication of sodium aluminate solution and carbonation-decomposing and followed by producing Al₂O₃ by roasting at high-temperature. After leaching out Si from fly ash, in the form of sodium aluminate by using NaOH solution with a concentration >40% (w/w), the leaching solution is diluted with water to facilitate separation of the sodium aluminate solution and the residue after alkali-leaching.

When producing silicon dioxide by a carbonating method from sodium aluminate solution, the obtained sodium aluminate solution can be basified with CaO or Ca(OH)₂, evaporating the dilute NaOH solution obtained to arrive at a concentration required for alkali-leaching in order for recycle-using.

The method according to the present invention breaks through traditional theory regarding recovery and desilication from high Si—Al-material, sets forth a novel process for producing Al₂O₃ by using high-Si—Al resources such as high-Al fly ash, in which silicon dioxide is recovered before recovery of Al₂O₃.

In comparison with the prior art, the present invention has the following advantages: 1) Prior art in the field of treatment of fly ash is only recovers Al₂O₃ from fly ash, with the remaining residue all used to produce cement, and with a low recovery rate of Al₂O₃ and low additional value of production. In contrast, the present invention recovers Si from fly ash by producing a Na₂SiO₃ solution and white carbon black, which have broad usage and significantly raise the added value of production, extending the industry chain of Si contained resources. Efficient recovery of Si from fly ash causes the Al—Si ratio of the residue after alkali-leaching to raise, making it a low-cost material for producing Al₂O₃, which raises the use value of fly ash. 2) The residue created after producing Al₂O₃ can be used to make either filler or cement. 3) All of the liquid phases of the process can be efficiently recycled, and no residue or harmful gas are released during the process. 4) The process is simple, with low capital investment, low operational cost and high added value, by which substantial elements of the fly ash can be fully used. It is a prosperous industry method for utilization of fly ash resources.

EXAMPLE 1

After roasting 100 kg of fly ash that has an average particle diameter ≦30 μm and contains Al₂O₃≧40% below the temperature of 400° C., it was put into a reaction kettle, adding into it 150 L of 80% NaOH solution and immersing for 1 h at 115-125° C. with stirring, then diluting by adding 200 L of water or thin alkali solution, to obtain 310 L of Na₂SiO₃ solution with 118 g/L SiO₂ and 65 kg residue after alkali-leaching with a Al—Si ratio of about 3.25. After diluting it from 118 g/L to 60 g/L, Na₂SiO₃ solution was moved into a carbonating kettle. It was filled with CO₂ gas, followed by carbonating with stirring at 70-85° C. until pH=9. Then carbonating was stopped followed by filtering and separating, to obtain about 35 kg of white carbon black, which contains >98.5% SiO₂. The carbonated filtered solution was mixed with CaO. After basifying at 75° C., it was evaporated till the concentration readied the concentration required by the alkali-leaching ash. Then it was sent back to the alkali-leaching steps for recycle use.

A slurry was produced by adding Ca according to CaO:SiO₂≦2 and by adding alkali according to Na₂O:Al₂O₃+Fe₂O₃≧1:1.1 into desilicated residue after alkali-leaching It was calcined at 950-1350° C. to get clinker. The clinker was dissolved and steeped with thin alkali solution, followed by separating solid from liquid. The desilicated solution was moved into a carbonation kettle, followed by filling CO₂ gas to carry out carbonized precipitation to precipitate Al(OH)₃. Filtering and separating was carried out when the solution was at a pH=8, to obtain 34 kg Al(OH)₃. After cleaning, Al(OH)₃ could be calcined into Al₂O₃. Residue after Al-recovery could be used to obtain filler with excellent performance by conventional technical treatment, or could be used to produce cement by conventional treatment

EXAMPLE 2

100 Kg of fly ash that had been roasted at 200° C. and had an average particle diameter ≦30 μm and contained Al₂O₃≧41% was put into a reaction kettle, followed by adding 160 L of 70% NaOH solution and boiled immersing for 2 h at 120° C. with stirring, then diluting by adding 200 L of water or thin alkali solution, resulting in 300 L Na₂SiO₃ solution containing 113 g/L SiO₂ and 66 kg of residue after alkali-leaching with an Al—Si ratio ≧3. The post-treatment of sodium carbonate solution and residue after alkali-leaching was the same as in Example 1.

EXAMPLE 3

100 Kg fly ash that had a particle diameter ≦50 μm was roasted at 850° C., followed by moving it into a alkali-leaching kettle after removing C. 220L of 60% NaOH solution was subsequently added into it, followed by immersing for 2.5 h at 125° C. with stirring, and then diluting by adding 200 L of thin alkali solution, resulting in 350 L of Na₂SiO₃ solution containing 110 g/L SiO₂ and about 65 kg of residue after alkali-leaching with a Al—Si ratio >4 obtained simultaneously. The post-treatment of sodium carbonate solution and residue after alkali-leaching was the same as in Example 1.

EXAMPLE 4

100 Kg of wet fly ash released from a power plant was directly bumped into a ball mill and ground into particles with diameter ≦30 μm. Water content in the ash was measured A solution of 70% (w/w) NaOH was then prepared, followed by immersing the fly ash for 2 h at 120-130° C. with stirring, and then diluting by adding 200 L of thin alkali solution. Obtained was 310 L of Na₂SiO₃ solution containing 103 g/L SiO₂ and about 66 kkKg of residue after alkali-leaching with a Al—Si ratio ≧3. The post-treatment of sodium carbonate solution and residue after alkali-leaching is the same to Example 1.

EXAMPLE 5

1 Ton of fly ash was ground and iron removed by magnetic separation then subsequently steeped in 10% H₂SO₄ solution for 240 h at ambient temperature. After filtering out the steeping solution, the remainder was washed until reaching pH=5, resulting in activated fly ash. The activated fly ash was moved into a reaction kettle, in which 1.6 t of 75% NaOH solution was added followed by boiling immersed for 2 h at 100° C. with stirring than diluting by adding 2.5 t water. The filtered solution was put into a carbonating kettle, in which CO₂ gas was filled subsequently. Carbonation with stirring was carried out at 80° C. till pH=9. After stopping of the carbonation filtering and separating was carried out, resulting in 420 kg of silica. Measurement showed that the purity is 98.7%. The post-treatment of sodium carbonate solution and residue after alkali-leaching was the same as Example 1.

EXAMPLE 6

1 Ton fly ash was ground and iron was removed by magnetic separation. It was subsequently steeped in 18% NaOH solution for 150 h at ambient temperature. Activated fly ash was obtained after filtering out the steeping solution. The activated fly ash was moved into reaction kettle, in which 2 t of 65% NaOH solution was added subsequently and followed by boiling immersed for 3 h at 110° C. with stirring, then diluting by adding 2.5 t water. The carbonating kettle was filled with filtering solution and CO₂ gas. Carbonation with stirring was carried out at 80° C. till pH=9. After stopping of the carbonation, filtering and separating was carried out, resulting in 400 kg of silica. Measurement showed that the purity was 98.5%. The post-treatment of dodium carbonate solution and residue after alkali-leaching was the same as Example 1.

EXAMPLE 7

1 Ton fly ash was ground and iron-removed by magnetic separation. It was subsequently steeped in 30% H₂SO₄ solution for 480 h at normal temperature, after filtering out the steeping solution, the remainder was washed until pH=5, resulting in activated fly ash. The activated fly ash was moved into reaction kettle and added with 1.6 t of 75% NaOH solution, and followed by boiled immersing for 3 h at 105° C. with stirring, then diluting by adding 2.5 t water. Into the carbonating kettle was filled with filtering solution and CO₂ gas. Carbonation with stirring was carried out at 80° C. till pH=9. After stopping of the carbonation, filtering and separating was carried out, resulting in 420 kg of silica. Measurement showed that the purity was 98.7%. The post-treatment of Sodium carbonate solution and residue after alkali-leaching was the same as Example 1.

The utility of the present invention is seen in the advantages of simplicity, low capital investment, low operating cost and high added-value making utilization of fly ash a economical resource.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 

1. A process for recovering silica and alumina from coal fly ash, comprising: a) alkali-leaching Si from fly ash at 70-150° C. with a NaOH solution of concentration >40% (w/w), then separating a Na₂SiO₃ solution from a residue having Al—Si ratio ≧2; b) concentrating the Na₂SiO₃ solution by vaporization, or obtaining silica by carbonation; and c) producing Al₂O₃ from the residue having Al—Si ratio ≧2.
 2. The process of claim 1, wherein the fly ash is activated by one or more of the following steps: a) roasting-activating the fly ash, b) steeping fly ash in H₂SO₄ solution, c) steeping fly ash in NaOH solution.
 3. The process of claim 1, wherein the fly ash is a high Al coal gangue, a kaolinite or middle-low-class bauxite and further comprises activating at 900-1100° C., immersing out SiO₂ with concentrate caustic alkali, and treated to produce alumina.
 4. The process of claim 2, wherein the fly ash is a high Al coal gangue, a kaolinite or middle-low-class bauxite, and further comprises the steps of activating at 900-1100° C., immersing out SiO₂ with concentrated caustic alkali, and treated to produce alumina.
 5. The process according to claim 1, further comprising producing filler or cement using the residue after the recovery of Al.
 6. The process according to claim 2, further comprising producing filler or cement using the residue after the recovery of Al.
 7. The process of claim 2, wherein the fly ash is a high Al coal gangue, a kaolinite or middle-low-class bauxite and said roasting-activating is carried out under the temperature of ≦950.
 8. The process of claim 2, wherein said H₂SO₄ solution is a H₂SO₄ solution of 10-30% (w/w) concentration.
 9. The process of claim 2, wherein said NaOH solution is a 5-20% (w/w) NaOH solution.
 10. The process of claim 1, wherein the Na₂SiO₃ solution is basified with CaO or Ca(OH)₂ to produce a NaOH solution for recycle-use. 